(lispref.info)Constant Variables

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Variables that Never Change

   Emacs Lisp has two special symbols, `nil' and `t', that always
evaluate to themselves.  These symbols cannot be rebound, nor can their
value cells be changed.  An attempt to change the value of `nil' or `t'
signals a `setting-constant' error.

     nil == 'nil
          => nil
     (setq nil 500)
     error--> Attempt to set constant symbol: nil

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